The research proposed here will address how physical factors control fundamental physiological processes, using marine macroalgae as a model system. The studies will be conducted in two coastal marine environments that support ecosystems that are beneficial to adjacent human populations, and are impacted increasingly by human activities in the coastal zone. These systems provide a range of variation in physical factors that can elucidate the role of organismal adaptation in fine tuning physiology to the physical environment. Achieving a better understanding of macroalgal physiology and its coupling with the changing physical environment may help predict, and perhaps mitigate, important changes in the structure and function of coastal marine environments; changes that often have cascading effects on human populations. Microalgae also are a convenient model system to address questions about physiological control because they are simple morphologically, respond relatively quickly to changes in their physical environment, and are manipulated easily, allowing experimentation. Using the principles of fluid biophysics and diffusion, and state-of-the-art instrumentation and experimentation, specific hypotheses are posed and will be tested about the control of physiological processes by the physical environment. These approaches Are the applicable equally to similar diffusion/mass transfer processes that occur in all fluid environments, including the exchange of materials between cells and the fluid matrix within animal tissues.